Scientific abstract Epilepsy affects over 60 millions of individuals worldwide. It is estimated that 30 to 40% of epilepsy patients have refractory epilepsy. Surgery may be offered to some patients who are refractory to medical therapy. While better seizure freedom rates are commonly achieved with comprehensive resective surgery, it may be associated with postoperative deficits depending on the area of resection. Pre-surgical evaluation often involves scalp EEG followed in some cases by intracranial monitoring techniques can help localizing seizure onset zone. These procedures are however are often very invasive and have potential for adverse events. High density EEG offers a potential non-invasive alternative to accurately localize epileptic onset zone in individual patients. To date, most high-density EEG studies focused on source reconstruction of scalp spike power [1] and did not take advantage of the high temporal resolution offered by EEG signals to explicitly study spike origin and propagation dynamics. Here we propose to use algorithms previously developed by our team to assess the cortical propagation of discrete EEG events such as sleep slow waves and spindles [2] to characterize the origins and propagation patterns of epileptic spikes captured using high-density EEG recordings. We plan to characterize reproducibility of origins and propagation patterns of epileptic spikes both at the scalp level and in source space, and compare the results obtained using high-density EEG to the ones obtained using intracranial recordings performed in the same patients. We will complement this study by directional connectivity assessments using Granger causality and Dynamic Causal Modeling. If successful, this approach will provide useful information to guide selective resection surgery in drug-refractory epileptic patients, potentially circumventing the need for invasive studies. 1